It’s July 11, or 7/11, the day each year that a certain convenience store chain celebrates as 7-Eleven Day. That’s the official name anyway. Most customers know July 11 simply as Free Slurpee Day.

In what’s become an annual event, 7-Eleven stores are giving away one free Slurpee per customer on July 11. Besides generating goodwill among customers, the event has proved curiously successful in boosting sales of Slurpees on the day of the giveaway.

It’s understandable that sales of other items spike on Free Slurpee Day, what with many customers picking up a Slim Jim or bag of chips to go with their free Slurpee. The increase in Slurpee sales on the day they’re handed out for free may come as more of a surprise. But apparently what happens is that customers often wind up wanting a bit more Slurpee than what’s provided in the free sample size, so they purchase a larger slushy drink after getting a complimentary taste.

While 7-Eleven is understandably sticking with 7/11 as its national giveaway day, the chain is getting rid of one part of its number-based marketing—and consumers win out as a result. Instead of going with a 7.11-ounce free Slurpee as it has in the past, this year 7-Eleven is simply giving out Slurpees in its usual Small size: 12 ounces.

Why the change? “Moms asked us for cups with lids,” Laura Gordon, vice president of marketing and brand innovation for 7-Eleven, told USA Today. The specially created 7.11-ounce cups don’t have lids, but the 12-ouncers do. “That means less mess in the car.”

It also means 7-Eleven doesn’t have to pay to produce special lids (or special sized cups) for the Slurpee giveaway, while simultaneously making customers happier by doling out a few more ounces of complimentary icy refreshment.

7-Eleven expects to give away roughly 7 million Slurpees on Thursday, July 11, 2013, up from around 4.5 million free Slurpees in 2011. Slurpee sales, meanwhile, may rise as much as 40% on Free Slurpee Day. Sales of hot dogs, beef jerky, and other snacks that pair well with icy drinks will surely soar as well.